Counterfeiting is the act of making an imitation (a counterfeit) of an object such as currency or private documents. Counterfeiting has been used with the intent to fraudulently pass the imitation off as the genuine object.
Anti-counterfeiting measures are constantly in development to protect both currency and certain documentation. As digital printing techniques emerged in the 80's and 90's, advances in digital color printing and increased resolutions have narrowed the technology gap between counterfeiters and law enforcement. The impact of this can be most easily seen in the redesigns of US Currency over the last ten years.
Additionally, new security techniques are constantly added such as security strips embedded in paper, use of holograms, color shifting ink as well as others. Each of these varies in effectiveness based on the technique and cost of implementation. The lifespan of their usefulness is also impacted by the advancement of reproduction technologies and how readily these become available to counterfeiters.